This invention concerns an apparatus for testing visual acuity or a so called retinometer, which makes possible, particularly when there is impaired refraction medium in an eye, the measuring of potential visual acuity, or clearness of vision.
A book by Bernhard Lachenmayr, "Potential Vision Sharpness With Impaired Refraction Medium" ("Potentielle Sehscharfe bei Storungen der brechenden Medien"), Quintessenz Publishers GmbH (Quintessenz-Verlags-GmbH), Munich, 1993, contains a summarized representation of apparatus known at that time for testing for visual acuity.
A knowledge of retinal visual acuity provides a valuable diagnostic and prognostic aid for an ophthalmologist when a medium is opaque, or clouded. This is particularly the case when it should be decided if an existing vision impairment is caused by opacities of the optical media alone or if vision functions, and information processing therefor, are also disturbed. For example, a cataract operation--replacement of an opaque lens of a patient with an artificial lens--is only practical if reduced visual acuity cannot also be traced to other disease changes.
It is common to previously-known retinometers that a line pattern or a vision test chart is formed on a retina of a patient's eye through a microscopically small area of a refraction medium which is not very opaque, or not opaque at all. This allows a testing substantially independent of impairment of the refraction medium. The smaller the line pattern, or the test characters, that the patient can identify, the better is his vision clearness, and visa versa.
In a retinometer of Rodenstock (see page 83, right column, where indicated) two coherent light beams are produced by reflecting a laser beam on a plano-parallel glass plate which produce, by interference, a line pattern on a retina of a patient. A spacing of the lines is dependent on a thickness of the glass plate. By rotating an intermediate prism the lines can be rotated.
In order to obtain a line pattern with sufficient contrast, high tolerance demands must be made on the glass plate and on the coherence of the laser as well as on guides for the beam path. Unavoidable three dimensional interference patterns (so called speckles) occurring at high line densities interfere with a patient's perception.
Because of optical principles and great optical and mechanical requirements, the device has a large structural length and is quite heavy so that it must usually be stationary. Operation of the laser requires an electrical power connection.
With a SITE-IRAS interferometer (page 88, right column, indicated portions, as well as a prospectus of the firm Interzeag) a microscopically narrow slit illuminated with a light and having a high magnifying objective forms a slit light source imaged just before a holographic grid. This divides the light into two coherent beams of equal intensity. Via a lens system, the two beams are projected into a patient's pupil and create on an eye background, by interference, black and white lines. A line spacing is determined by a position of the movable phase grating along a length of the optical axis, the direction of the lines being changed by simultaneous rotation of the slit and the phase grating.
A contrast of the line patterns depends, on the one hand, on an optical quality of the holographic phase grating, which requires a great expense. On the other hand, use of white light leads to unavoidable spectral refraction, which causes a deterioration of contrast. The axial positioning of the phase grating and parallel rotation of the slit requires great mechanical precision.
This known device can be held in a hand during an examination, however, optical principles require that it have a relatively long structure so that it is uncomfortable to handle. An external power supply is necessary for the lamp which is built into the apparatus, thereby requiring a power hook-up.
A device conceived by Lotmar (page 85, right column, indicated portions) employs two opposingly rotatable, closely-adjacent reticles, or diffraction gratings or line plates. Upon illumination of the recticles with a lamp through a very small slit or a small pin-hole diaphragm, diffraction spectrums arise according to Moire principles, from which, by means of diaphragms, two adjacent coherent beams of the same order and therefore the same intensity can be separated. By using white light, instead of almost point-like diffraction peaks, spectrally separated diffraction distributions are produced. With a dispersion prism in a beam path the spectral divisions can be united again into white light, so that a black and white line pattern is developed on a retina. By rotation of the two line plates through very small angles, line spacing can be changed while the direction of the line pattern can be moved by the prism.
The optical principles of this device require a great structural length and a very large expense for necessary precision to opposingly rotate the line plates, or discs, through different size angles in fine steps. Application of Moire-principles requires an extremely high intensity lamp whereby only a commercial power hook-up operation is possible. Thus, this device is also operated from a stationary position.
With the Potential Acuity Meter (PAM) of the firm Mentor (page 72, right column, indicated portions) a smaller chart with test characters of various sizes is projected onto a retina. For correcting refraction errors of an examined eye, the positions of the chart can be moved axially by a rotation knob.
To avoid unclearness caused by diffraction, which particularly disturbs the contrast of small optical characters and thereby makes them difficult to recognize, use of larger test characters, or marks, is required. Because image sharpness of projection systems is distance sensitive, the optical system of PAM must be adjusted for refraction of an examined eye by respectively moving the test characters so as not to get false results from the visual examination. This requires a relatively large mechanical expense, a large structural length, and a high light intensity so that the device must be stationary and coupled to a commercial power source.
A requirement for optically adjusting this device requires an additional expenditure of time and work for difficult patients at a beginning of an examination as well as during it. A correction for astigmatic refraction is not provided for by this device and must be handled by a patient's eyeglasses or other eyeglasses.
Known retinometers are, with the exception of the SITE IRAS device, hindered by their structural size, since they are only suitable to be placed in operation at stationary sites. In order to carry out a visual acuity examination, a patient must come to the device and to the doctor, which, for example, is impossible for those in bed or who are immobile for other reasons. A stationary structure is however also disadvantageous for mobile patients as well because they are required to spend many minutes in bent-over or uncomfortable sitting positions, which makes it difficult, particularly for older persons who form the majority of patients. The only known non-stationary device (SITE IRAS) has a relatively large structural length and requires also a power connection so that it only has limited mobility.
It is an object of this invention to provide an apparatus for testing for visual acuity having a structural length which is small and which does not have undue requirements for precision of optical and mechanical elements.